The present invention relates to lift pins used for dechucking substrates held by electrostatic chucks in process chambers.
In semiconductor processing, lift pins are used to lift and lower a semiconductor substrate onto a chuck that is used to hold the substrate in a process chamber. Typically, a robotic arm transports the substrate into an upper portion of the process chamber where the substrate is deposited on lift pins that extend upwardly through the chuck. The lift pins are then lowered into a lower portion of the process chamber to deposit the substrate on the chuck. Thereafter, the robotic arm is withdrawn from the chamber.
Electrostatic chucks are often used to electrostatically attract and hold the substrate in the process chamber during processing of the substrate. Electrostatic chucks are either monopolar or bipolar chucks. A monopolar electrostatic chuck has a single electrode which operates in conjunction with a plasma formed in the process chamber to accumulate opposing electrostatic charge in the substrate and the electrode. A bipolar chuck can be used in non-plasma processes, and typically includes two or more electrodes which are maintained at opposing electric potentials to induce opposing electrostatic charge in the electrodes and substrate. The opposing electrostatic charges in the substrate and the electrodes of the chuck cause the substrate to be electrostatically held to the chuck. Typically, the electrodes of the chuck are electrically biased with respect to the substrate by a DC voltage or a low frequency AC voltage. Also, typically, the plasma in the process chamber is formed by (i) inductive coupling by applying a high frequency RF current to an inductor coil wound around the chamber, (ii) capacitive coupling using process electrodes in the chamber, or (iii) both inductive and capacitive coupling.
After processing of the substrate, the DC voltage applied to the chuck electrode is terminated to release the substrate, and the lift pins are raised upward through holes in the chuck to lift the substrate off the chuck by pushing up against the substrate. The robotic arm is then reinserted below the substrate to withdraw the processed substrate from the chamber.
One problem with conventional lift pins arises when the lift pins attempt to lift the substrate off the chuck. Residual electrostatic charge in the substrate generates attractive electrostatic forces between the substrate and the chuck, that cause the substrate to adhere to the chuck, even when the voltage to the chuck is terminated. The upwardly pushing lift pins can damage or break the substrate. This problem is worse when the lift pins are made of ceramic material. The electrically insulative ceramic pins trap residual electrostatic charge in the substrate, causing the substrate to stick to the chuck as the lift pins are pushed against the substrate, resulting in damage or breakage of the substrate.
One solution is to use electrically grounded metal lift pins that allow the residual charge in the substrate to discharge through the lift pins. However, the metal pins also allow the high frequency RF currents used to generate the plasma in the chamber, and to attract the plasma to the substrate, to propagate through the metal pins and into the lower portion of the chamber. This results in plasma formation in the lower portion of the chamber, causing erosion of the metal parts therein, and wasting the power used to generate the plasma. Also, the plasma heats up the lower portion of the process chamber, and causes deposits to form on the components therein.
Therefore, there is a need for lift pins which allow discharging of residual electrostatic charge in a substrate to prevent damage or breakage of the substrate during dechucking of the substrate. It is further desirable for the lift pins to reduce propagation of the RF currents used to generate a plasma into the lower portion of the process chamber. The present invention satisfies these needs.